One aspect of the present invention is directed towards an electro-optic device allowing continuous wavelength tuning.
Current efforts to replace paper include the utilization of new materials such as electronic ink. Based on this technology, electrically switched dyes bound in microscopic containers are suspended in a solvent. When switched, colored dyes physically move to the top surface of the microscopic containers where they are used to reflect part of the visible spectrum back to an observer""s eye; a plurality of microscopic containers collectively create an image. Multiple dyes are used to cover the visible spectrum, however, light utilization is typically poor. The creation of images using these reflective/absorptive dyes, therefore, still needs considerable development.
Another technique for creating displays involves the use of electrically-variable stratified mediums to create a dynamic version of the imaging technique developed by Lippmann at the end of the last century. Basically, color is extracted when light is partially transmitted and reflected from multiple layers in a stratified medium, where the spacing of the layers dictates which color resonates. This interference phenomenon can be quite efficient. However, there is a need to develop materials and processes such that the spacing of the layers within the material volume can be varied electronically. Unfortunately, the piezoelectric materials under investigation require high voltages (large power) and have no long term persistence, i.e., they relax to their static state when power is removed. Moreover, a complex manufacturing process is required to produce the multiple layers composed of uncommon materials which typically results in poor yields and high cost.
Another technology for producing electronic paper type displays currently under development involves the use of reflective liquid crystal displays (LCDs). Here, the application is leveraged off the huge flat-panel display industry. One group of investigators has demonstrated time-stable, virtually zero-power consumption LCDs that are produced using standard nematic phase liquid crystals aligned with an underlying surface structure. When the liquid crystal molecules align to the surface structure, they produce domains within a liquid cell that are mechanically robust. Thus, mild vibrations and other physical shocks typically do not disturb the alignment. Optical effects of the surface structure (typically just an asymmetric profile grating) are not exploited, and are in fact suppressed due to their typically broad spectrum dispersive nature.
A second group of investigators has begun exploiting the optical effects of a surface structure grating to direct a portion of the broad spectrum reflected from the grating through a pixel window in an LCD. Each pixel consists of red, green, and blue sub-pixels, wherein the sub-pixels include a grating with a pitch such that the diffraction angle of the dispersed light matches the acceptance angle of the pixel window. Liquid crystal is used as a shutter over each window or pixel to block light.
Another class of researchers works with polymer dispersed liquid crystals, where a volume holographic medium such as a photo-polymer is infused with liquid crystal material. The liquid crystal collects in pockets within the photo-polymer and has no particular alignment when an electric field is not present. A volume hologram is then recorded within the medium using a diffusing screen as the object in a manner similar to that used in the manufacture of high-efficiency, non-specular reflecting diffusers. Most of the liquid crystal pockets migrate to areas where the recording intensity is weak, further accentuating the resulting stratification comprising the volume hologram. When a high-voltage is applied across the medium, the liquid crystals in the pockets align with the electric field, and the light reflected from the hologram is then predominantly scattered by the liquid crystal pockets. The result is a device that can be switched between a high brightness single color reflection and a milky white reflection. Stacking three such devices with red, green, and blue reflections yields a full color display. Unfortunately, the design requires a high voltage operation and includes having to write three separate volume holograms in production.
Lastly, researchers at Kent Displays (Kent State University) have begun to exploit the benefits of recently discovered properties of cholesteric liquid crystals. Apparently, they have been able to fabricate liquid crystal cells where the liquid crystal molecules can be made to organize and align in planes parallel to the cell walls. This creates a stratified medium with layers spaced in the 200 to 300 nm range depending on the liquid crystal composition. These layers will reflect visible light in the 400 to 600 nm range in exactly the same manner as that described by Lippmann. When voltage is applied across the cell, the planar structure is destroyed as all the molecules align parallel with the electric field. The liquid crystal is then optically transparent, reflecting the natural color of the cell substrate. As with the polymer dispersed LCD efforts, full color is attained by stacking three liquid crystal cells reflecting either red, green, or blue.
It is an advancement in the art to provide an electro-optic device capable of continuous wavelength tuning of reflected or transmitted incident electromagnetic waves.
According to the principles of the present invention, an electro-optical device comprises a substrate having successive reflective steps spaced from an electrode, where the steps reflect incident electromagnetic waves such as broadband light. A voltage applied to an electrode disposed adjacent to the substrate adjusts the wavelength of the electromagnetic waves in the space between the substrate and the electrode. Accordingly, the voltage applied to an electrode such as a transparent conductive strip adjusts a wavelength or band of wavelengths that is reflected to an observer.
In one application, the space between the electrode and substrate includes a material having a variable index of refraction that is adjusted based on an applied voltage. Such a material filling this space can be, for example, a liquid crystal. Preferably, the material is disposed so that it is in communication with the reflective steps of the substrate.
Based on these principles, an image can be produced for an observer by adjusting light reflected from multiple reflective steps of the substrate. To enhance a viewing field of reflected wavelengths of light for an observer, the reflective steps can be angled with respect to a planar axis of the substrate to reduce potential glare.
Another embodiment according to the principles of the present invention is directed to an apparatus and method for tuning which of multiple incident electromagnetic waves pass through an electro-optic device. The electro-optic device comprises a first transparent substrate having terraced steps on a face. A second transparent substrate also having terraced steps is disposed to face the terraced steps of the first substrate. A material, such as liquid crystal, having a variable index of refraction is provided between the first and second substrate to adjust wavelengths of passing light. Based on a tuned index of refraction of the material disposed between the first and second substrate, certain wavelengths pass through material and both substrates of the electro-optic device. Wavelengths of light corresponding to a spacing of terraced steps of the first and second substrate become resonant between the first and second substrate and are reflected out an end of the electro-optic device into which the wavelength of light was originally directed.
The electro-optic devices according to the principles of the present invention are advantageous because wavelengths of light can be selected dynamically based on a controlling input. Other optical devices such as thin film filters are limited to a preselected wavelength as set at a factory.
Based on the principles of the present invention, it is possible to manufacture flexible, inexpensive substrates that can be used in display applications. Such displays require little or no power to maintain a displayed image, make efficient use of ambient room light for enhanced viewing by an observer, and can be produced as flexible substrates so that a display can bend without being damaged. Additionally, the displays can be manufactured lightweight and thin for space-sensitive applications.